In recent years, as a negative electrode active material for a non-aqueous electrolyte secondary battery, negative electrode materials containing an element capable of storing and emitting lithium ions reversibly and electrochemically such as Si (silicon) or Sn (tin) have been drawing attention. Using such a metallic element as a negative electrode active material may achieve a larger negative electrode capacity than using carbon materials such as graphite, which are typical conventional negative electrode active materials. Silicon, for example, has a theoretical discharge capacity of about 4199 mAh/g, which is about 11 times larger than that of graphite.
These negative electrode materials, however, tend to be greatly expanded as a result that these negative electrode materials form an alloy with lithium (Li) and change their structure when storing lithium ions. In a case where graphite is used as a negative electrode active material, lithium ions are intercalated into the interlayers of graphite. This intercalation reaction reduces the volume expansion caused by the storage of lithium ions to about 1.1 times. In contrast, if Si stores lithium ions to its maximum capacity, the negative electrode active material is theoretically expanded about 4 times more than before the storage. When the negative electrode active material is greatly expanded due to the storage of lithium ions in this manner, the active material particles may be broken or the active material layer may be peeled off from the current collector layer, thereby decreasing the conductivity in the negative electrode. The decrease in the conductivity of the negative electrode leads to a decrease in battery characteristics such as charge-discharge cycle characteristics.
The peeling off of the active material layer can be prevented, for example, by increasing the proportion of a binder in the active material layer. However, this may decrease the negative electrode capacity because the binder does not contribute to charge-discharge reactions.
Under such circumstances, various techniques have been proposed in order to reduce the destruction of the active material layer or a decrease in conductivity due to the expansion of the active material in a negative electrode that uses a high-capacity material such as Si as an active material, which stores lithium ions.
For example, Japanese Patent Unexamined Publication No. 2002-260637 discloses a negative electrode which is formed by sintering a mixture of Si-containing active material particles and conductive metal powder on the surface of a current collector in a non-oxidizing atmosphere. The current collector is composed of a metal foil or a conductive metal powder which are made of copper or a copper alloy.
In this negative electrode, however, the sintering process of manufacturing causes the generation of a Cu—Si compound which does not electrochemically react with Li, thereby decreasing the negative electrode capacity. Furthermore, the sintering is required to be performed at high temperatures, making it likely that the copper used in the current collector is melted or hardened. Such phenomena may destroy the flexibility of the current collector, thereby interfering with the formation of the electrode assembly.
Japanese Patent Unexamined Publication No. 2004-127561, on the other hand, discloses a negative electrode including a current collector and a thin film formed thereon. The current collector is made of a material that does not form an alloy with Li, and the thin film is made of a metal that forms an alloy with Li or of an alloy containing the metal. In this negative electrode, a negative electrode active material layer having protrusions and depressions is formed selectively in a predetermined pattern on the current collector by photoresist, plating, and the like. The protrusions of the negative electrode active material are columnar and surrounded by spaces that absorb the volume expansion and avoid the destruction of the negative electrode active material. This patent publication further discloses a secondary battery using the negative electrode which includes a current collector and a negative electrode active material layer formed in a pattern having protrusions and depressions on the current collector. The negative electrode active material layer is faced with a positive electrode via a separator interposed therebetween in the same manner as in the conventional batteries.
However, the manufacturing method of a negative electrode thus structured requires the formation of a photoresist mask used to pattern the negative electrode active material layer. This complicated pretreatment leads to a reduction in productivity.
Moreover, the current collector is required to have strength sufficient to be handled in these manufacturing processes. Therefore, the current collector should have a thickness over several micrometers corresponding to 50% or more of the thickness of the negative electrode active material layer, which decreases the volumetric efficiency or packing efficiency in the battery. As it gets thicker, the current collector becomes less flexible. Therefore, when the negative electrode active material layer is repeatedly expanded and contracted during charge and discharge, the negative electrode active material and the current collector are likely to be peeled off from each other. As a result, the current collector becomes hard to hold the negative electrode, thereby reducing current collection performance.